Photonic circuits find increasing use in computing devices. The use of optical signals in device communication has significant potential advantages over electrical communication, namely in terms of power and bandwidth. However, many practical implementations of optical communication are still lacking One of the primary difficulties facing the use of optical signals has to do with scaling of the optical configuration, especially in converting between optical to electrical signals.
FIG. 1 is a block diagram of a prior art system with a photodetector disposed over a waveguide channel. Device 100 is shown from a top view, looking from above a semiconductor structure. Device 100 includes a large SOI (silicon on insulator) waveguide photodetector 120. The cross section area of photodetector 120 is in the range of 10 μm2 to 30 μm2 and has a mechanism that allows source light 102 to be vertically reflected onto photodetector 120. Source light 102 propagates through a channel formed by oxide 132 and oxide 134, until vertically reflected to a Ge diode on top of the reflected light spot to be absorbed and converted to an electrical signal.
While the structure of device 100 provides responsivity and bandwidth higher than traditional devices (>0.8 A/W for responsivity and >18 GHz for bandwidth), further performance scaling is limited. Photodetector 120 operates to provide an electrical signal based on contacts 112 and 114. The distance between the two contact vias is relatively long—no shorter than 20/sin(2×54.7)/2=10.6 μm for 20 μm thick SOI. Such a long length creates a large resistance in the conduction path from photodetector 120 to an associated contact 114 that connects to the Si of the waveguide. The bandwidth of device 100 is therefore strongly constrained by the RC (resistive-capacitive) characteristics of the device.
The RC characteristic can be reduced by reducing the size of photodetector 120, but that is expected to rapidly decrease the responsivity of device 100 due to a fixed design rule in-between the photodetector and the oxide trenches 132 and 134. Attempting to place contact 114 closer to contact 112 by lateral placement instead of longitudinal placement is ineffective due to the oxide filled trench. A conduction path between the contacts cannot be created without major modifications to device 100. For example, fabrication processes would be greatly complicated by use of a poly shunt and a segmented waveguide. The poly shunt would be in place of part of the oxide in the trench, replaced with amorphous Si, for example. Such an approach inevitably introduces topology and may raise various processing issues afterwards. Also, the resistance of a poly shunt can be large without appropriate design/process fine tune. The segmented waveguide would result in the waveguide not being fully confined by the oxide, which is expected to reduce responsivity.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.